2-Aryl-3-hydroxyquinolones, a new class of dyes with solvent dependent dual emission due to excited state intramolecular proton transfer

Abstract

Herein, the fluorescence properties of a series of 2-aryl-3-hydroxyquinolones (3HQs) were investigated and compared with the properties of well-studied 3-hydroxyflavone. All these compounds were found to display dual fluorescence with well-separated bands in organic solvents and aqueous solutions. Using steady-state and time-resolved fluorescence spectroscopy, we showed that their dual fluorescence is due to an excited state intramolecular proton transfer reaction. Moreover, the absorption spectra of most 3HQs tested were found to be similar, indicating that they are not sensitive to the substituent at the 2-aryl ring. This was related by quantum chemical calculations to the non-planarity of these molecules which prevents conjugation between the two aromatic moieties. The only exception was the 3HQ derivative with a thiophene ring at position 2 which exhibited a red-shifted spectrum due to its more planar structure. In sharp contrast, the emission spectra and especially the intensity ratio of the two emission bands were highly dependent on the substituents at the 2-aryl ring and at the heterocyclic nitrogen. Moreover, N-methyl substituted 3HQs (N–Me 3HQs) demonstrate strong solvatochromic properties, with large changes in their fluorescence band intensity ratio as a function of the solvent polarity. In addition, the logarithm of these intensity ratios varied linearly with the Hammett constant associated with the substituent in the 2-aryl ring, enabling the design of 3HQ dyes with optimized intensity ratios in a given range of solvent polarities. Thus, 3HQs preserve the unique properties of 3-hydroxyflavones, namely dual emission that is highly sensitive to solvent polarity and to chemical substituents. Moreover, in comparison to 3-hydroxyflavones, 3HQ dyes exhibit higher fluorescence quantum yields and 10-fold increased photostability. These properties of the 3HQ derivatives make them prospective candidates for application as polarity-sensitive fluorescent labels for biomolecules.